Recurring slope lineae (RSL) in Newton Crater, Mars
Recurring slope lineae are narrow (0.5-5 m wide), relatively dark-toned features that form on steep (25-40˚), southern-hemisphere slopes, and that appear in early spring, grow longer in the downslope direction during spring and summer, and fade during autumn and winter.
Here is a comparison of the same crater wall on two different dates.
NASA / JPL / UA / Joe Levy
Recurring slope lineae at Newton Crater, Mars, in the early season (left) and the late season (right)
Recurring slope lineae flow downhill from the rocky cliffs at right to the sandy slopes at left. White bars shown the distance that individual lineae have flowed. Left image is a portion of ESP_022267_1380. Right image is a portion of ESP_022689_1380.
This image is in the public domain.
Original image data dated on or about May 30, 2011